The present invention relates a Hall-effect liquid level sensing apparatus and method. In the prior art, there has always been a need to determine the level of liquids which are stored in sealed tanks. Often, the environment within such a sealed tank is highly corrosive and dangerous including the formation of lethal gases and corrosive chemicals which would cause damage to conventional level sensing devices.
One intended environment of use of the present invention consists of the interior of a tank containing petroleum products such as oil. Mechanical float-type gauges have been used in this environment with considerable shortcomings due to the corrosive effects of the petroleum products as well as gases such as H.sub.2 S which are formed in such petroleum product tanks. Mechanical float-type gauges have problems in durability in such an environment because they include complex pivot points, pulleys in some cases and other moving parts which are susceptible to wear, corrosion, etc.
Another method of level measuring which has been used in the intended environment of use consists of a float having a magnet associated therewith and the use of indirect reading through the use of limit switches, reed switches, and other devices to determine the level of the magnet and thereby the float. However, the prior art does not teach such a device which may be used for continuous measuring purposes. Furthermore, it is well known to utilize ultrasonic level sensing devices in storage tank measurements but in the environment of a petroleum storage tank wherein stratification of formed gases may occur, inaccuracies may exist since there is no way to accurately know the precise gaseous environment which is present within the tank above the liquid level and therefore the precise speed of the sound waves which will be created by the ultrasonic device may not be accurately measured. Since ultrasonic level measuring devices depend upon knowledge of the speed of sound in the particular environment used, such devices may become impractical in the petroleum storage environment.
An additional method of oil tank level measurement uses the differential pressure also known as the hydrostatic head for oil level measurements. The problem with this sort of level measuring device is that as these devices have been constituted in the prior art, one may never know for sure if the pressure sensor is immersed within the oil or whether it is immersed within another liquid which has accumulated below the oil level. Thus, a need has developed not only to provide a level sensor which overcomes the deficiency in the differential pressure method and apparatus, but also the need has developed to provide a device which may be used in conjunction with a level measuring device to monitor changes in the specific gravity of the oil within the tank.
A further apparatus and method which has seen wide usage in the measurement of the liquid level in a petroleum products tank consists of hand-measurement with a mechanical tape. This apparatus and method have serious disadvantages since they are time consuming, subject to errors by the operator, rely upon hand-written recording and expose the operator to any toxic gases which may be present in the tank.
The following prior art is known to Applicants:
U.S. Pat. No. 3,316,763 to DeGiers discloses a fluid level gauge having a pivoted float which when it pivots reciprocates a magnetic actuator which results in the reciprocation of follower permanent magnets to thereby result in measurement of the liquid level in the container. The mechanical interaction which must necessarily occur in the operation of the DeGiers device may easily be damaged and compromised in the corrosive environment of a petroleum products tank and accordingly the DeGiers teaching is believed to be deficient vis-a-vis the present invention.
U.S. Pat. No. 3,505,869 to Crawford discloses a liquid level sensing device including a piston member having attached thereto a magnet which is attached at a position adjacent one of the walls of the container, which wall has mounted thereon a plurality of magnetic reed switches. With this device, the level of liquid within the tank may be determined through knowledge of which one of the reed switches is closed. The accuracy of the Crawford device depends upon how closely spaced the magnetic reed switches are, and the closer they are spaced, the greater the likelihood that the single magnet may inadvertently actuate more than one switch simultaneously thereby compromising the accuracy of the device.
U.S. Pat. No. 3,935,741 to Zinsmeyer, et al. discloses a liquid level sensor which is able to sense the levels of two immiscible liquids in a tank. The device includes a first magnet carried by a float on top of one liquid and a second magnet carried by a float on top of the other liquid. The first magnet operates a switch to indicate when one liquid reaches a predetermined level and the second magnet moves a coded tape with respect to a read head in accordance with changes in the level of the other liquid. This device is quite complicated in design and the transmitter used requires a quite complicated operation sequence in its operation.
U.S. Pat. No. 3,964,312 to Sebek disclosed a magnetically actuated fluid level indicator wherein a float is provided with a magnet and the float is contained within a tube for reciprocation therein. On the side of the tube is mounted an indicating device including a plurality of indicator platelets which change their position of rotation as a result of the effect of the magnetic field lines thereon. Thus, the experienced operator may easily tell the specific height of the float within the tube by merely observing the postion of the platelets. This device is extremely cumbersome in design, requiring a plurality of platelets which may be expensive. Furthermore, this invention requires that the platelets be mounted in an area where they may be visually observed while being close enough to the magnets on the float to ensure the transmission of the magnetic field thereto. Thus, in the environment contemplated by Applicants, a thin wall would have to be provided in the petroleum tank, thereby creating the possibility of leakage or other system malfunction.
U.S. Pat. No. 4,078,430 to Pemberton, et al. discloses a dual liquid level indicating gauge which may measure the levels of two liquids contained in the same tank. The device includes two floats and each float has a measuring tape controlled thereby to provide an indication of the liquid level through a single viewing window above the tank. The requirement for two separate indicating tapes and the further requirement of a viewing window exterior of the tank renders this device extremely cumbersome and difficult to install. Furthermore, the fact that the measuring tapes are always immersed within the liquid increases the possiblity of malfunction due to the corrosive effects of the environment therein.
U.S. Pat. No. 4,227,171 to Masuda discloses a liquid level sensing means including a magnet for applying magnetic flux to a galvano-magneto effect transducing means with changes in the flux resulting from changes in the liquid level. Of course, the present invention differs from the invention of Masuda since the present invention includes the provision of the selective lowering of a Hall-effect device on a carefully calibrated tape to measure the liquid level.
U.S Pat. No. 4,284,904 to Tetro discloses a liquid level measuring device wherein a tape has a plurality of encapsulated network units assembled thereto and the tape may be utilized to follow a float carrying a permanent magnet so that the liquid level in the tank may be determined. The present invention differs from this device since in the present invention only a single Hall-effect sensor is used and the tape to which the Hall-effect sensor is attached is selectively actuated rather than continuously actuated so as to increase tape life by avoiding the corrosive effects of the tank environment while reducing the frequency of movement and wear.
U.S. Pat. No. 4,356,729 to Kubota, et al. discloses a fluid level detector including a float having a magnet attached thereto and wherein when the float contacts a stop, a switch is closed to thereby indicate the lowermost position of the float. The device of Kubota, et al. is believed to merely consist of a safety device which enables one to determine wben the liquid level is at its lowest point. Of course, the present invention is much more detailed than the Kubota, et al. device, utilizing a Hall-effect device reciprocable on a tape to measure the level of a liquid within the tank.
U.S. Pat. Nos. 4,361,835 to Negy and 4,446,284 to Dummary each disclose liquid level detecting devices wherein a float or floats is/are reciprocably mounted over a tube having contained therein a column of sequential Hall-cells with each float having a toroidal magnet mounted thereon. The Hall-effect devices are actuated by the toroidal magnet of each float to indicate liquid level. The present invention has several advantages over the teachings of these patents, including the fact that only a single Hall-effect device is used and is selectively reciprocated within a tube about which a toroidal float is mounted having a permanent magnet contained thereon. The present invention is much simpler than the teachings of these patents and much more inexpensive and practical to install.
U.S. Pat. No. 4,425,796 to Boucher, et al. discloses a hermetically sealed tank gauge apparatus wherein a float is attached to a chain having a weight at its other end with the chain being attached to a gear so that movements of the float result in rotations of the gear and from the rotations of the gear the level of the fluid within the tank may be recorded. While this device has some similarities with the teachings of the present invention, especially in the monitoring of the rotations of a rotary member, the Boucher, et al. device is believed to be deficient vis-a-vis the teachings of the present invention in that unreliability may be attendant in a system wherein the corrosive effects of the fluid within the tank are continuously exposed to the mechanism in question. Furthermore, these corrosive effects may change the weight and other configurations of the float and/or weight to thereby cause resultant inaccuracies in the readings which may be obtained therefrom.
U.S. Pat. No. 4,467,647 to Yamane, et al. discloses a direct reading tank gauge having a tape of plastic or thin metal in an endless loop with an armature fixedly mounted to the tape. A tank float is magnetically coupled to the armature so that changes in the fluid level in the tank result in movements of the armature and thereby the tape, resulting in liquid level readings. Of course, the present invention differs from the teachings of Yamane, et al. for many reasons, especially in that there is no positive mechanical interaction between the float and the level measuring mechanism of the present invention but, rather, the passive interaction therebetween will result in increased accuracy in the readings obtained thereby.
U.S. Pat. No. 4,480,469 to Tyce discloses an adjustable differential fluid level float indicator which includes a float reciprocably mounted on a transparent tube with the float having a permanent magnet providing a magnetic field which may actuate magnetically sensitive switches within the tube to thereby enable the reading of the liquid level within the tank. While some similarities exist between the teachings of Tyce and those of the present invention, the present invention is quite advantageous over the teachings of Tyce as providing a device which enables the liquid level to be accurately determined without reliance upon the positions of a finite number of switches.
U.S. Pat. No. 4,532,800 to Coleman discloses a level indicator wherein a magnetic switch may be actuated by a permanent magnet carried by a follower to determine liquid level in the tank. Of course, the follower structure is quite cumbersome and since it is always exposed to the corrosive environment of the fluid and gases which may be in the tank, its reliability may be questionable in the long term.
In light of the prior art discussed above, and in view of the needs which have been developed for an inexpensive trouble-free and extremely accurate level sensing device which may be utilized in the corrosive environment of the interior of a tank containing petroleum distillates, the present invention has been devised so as to overcome the deficiencies and short comings of the prior art as delineated above.